Green Nanowire Synthesis

Green chemistry, also called sustainable chemistry aims at designing products and processes that minimize the use and generation of hazardous substances.

Being environmentally benign, the principles of green chemistry is being progressively applied in material sciences for manufacturing materials including nanoparticles, catalyzing chemical reactions, treating waste and generating energy.

With a plethora of innovations in the field of nanotechnology, nanoparticles are increasingly being used in many products and processes. Nanoparticles are of particular interest as they are effectively a bridge between bulk materials and atomic or molecular structures. Unlike bulk materials, their small size and correspondingly high surface area make them very reactive.

Nanoparticles of commercial interest include manganese oxides based nanostructures (nanospheres, nanorods and nanowires to name a few) that have attracted attention due to their wide range of applications in different fields such as catalysis, ion-sieves, rechargeable batteries, chemical sensing devices, and microelectronics. Several methods have been developed for the synthesis of manganese oxide nanowires; however most synthesis methods entail hydrothermal techniques that are often carried out at high temperature and pressure in the presence of solvents, concentrated acids, bases, and templates. A few of the major disadvantages of high-temperature nanowire synthesis methods include their high cost of fabrication (due to high energy inputs) and scale-up challenges to produce larger amounts of nanowires. Various purported greener syntheses of manganese oxide nanowires, via sol–gel processes and ionic liquids still involve post-treatment at higher temperatures thereby involving energy inputs similar to energy-intensive hydrothermal synthesis.

As an alternative to such methods, we synthesized manganese oxide nanowires in an aqueous matrix solely from a solution of manganese [Mn(II)] and an iron oxide catalyst (α-Fe2O3, also known as hematite) nanocrystals in the presence of a classic biochemical buffer, PIPES at room temperature and neutral pH. The end product of Mn(II) oxidation was Mn3O4 (hausmannite), a mixed valent Mn(II)/Mn(III) oxide.

In addition to using abundant or renewable raw source materials, as well as low toxicity, a frequently important factor in the development of sustainable nanotechnologies is the ability to produce nanomaterials according to green chemistry principles. With a limited number of precursor ingredients, water as the primary solvent, minimal energy input and minimal waste output, these results suggest an exciting approach in green nanowire synthesis to be further explored. Taken together, such green chemistry nanomaterial synthesis will not only pay off in a cleaner environment, but holds tremendous potential in the long run towards the more productive use of raw materials, a greater emphasis on renewable resources and potential energy independence.

About the author:

Dr. Harish Veeramani is presently a postdoctoral associate at the University of Waterloo in Ontario, Canada. This work was carried out during his term at Virginia Tech under the guidance of Dr. Michael F. Hochella Jr.